一个算计器的实现源码., 可以进行一般的运算

				/*
				 * function.c       
				 * Original Author: zhough@ruijie.com.cn, 2007-8-15    
				 *   文件功能的简要说明
				 * 该文件的功能是：该文件包含了四个函数，分别是：
				 *int getch(void); 取一个字符（可能是压回的字符）
				 *void ungetch(int); 把字符压回到输入中。
				 *int getop(char []); 获取下一个运算符或数值操作数
				 *void push(double f);把f压入到值栈中
				 *double pop(void);弹出并返回栈顶值
				*/
				
				
				#include 
				#include   
				#include 
				#include "function.h"
				
				
				#define BUFSIZE 100
				#define NUMBER '0'      /* signal that a number was found */
				#define MAXVAL 100 /* maximum depth of val stack */
				
				 
				int flag=0; 
				int bufp = 0;
				int sp = 0; /* next free stack position */
				
				char buf[BUFSIZE]; /* buffer for ungetch */
				double val[MAXVAL]; /* value stack */
				   
				   
				/* push: push f onto value stack */
				void push(double f)
				{
				   if (sp < MAXVAL)
				     val[sp++] = f;
				   else
				    printf("error: stack full, can't push %g\n", f);
				}
				
				
				/* pop: pop and return top value from stack */
				double pop(void)
				{
						  if (sp > 0)
						    return val[--sp];
						  else
						 {
						   printf("error: stack empty\n");
						  return 0.0;
						 }
				}
				
				
				void ungetch(int c)    /* push character back on input */
				{
						if (bufp >= BUFSIZE)
						  printf("ungetch: too many characters\n");
						else
						  buf[bufp++] = c;
				}
				
				
				int getch(void) /* get a (possibly pushed-back) character */
				{
				  return (bufp > 0) ? buf[--bufp] : getchar();
				}
				
				
				/* getop: get next character or numeric operand */
				int getop(char s[])
				{
						  int i, c;
						  /*删除表达式前面的‘ ’ 和’/t‘字符*/
						  while ((s[0] = c = getch()) == ' ' || c == '\t')
						              ;
						 s[1] = '\0';
						 	 
						  /*考虑有负数存在的情况*/
						   if(c=='-')
						    {
								    	  	/*判断它的下一个是否是数字*/
								    	  	  c=getch();
								    	    /* isdigit()功能:判断字符c是否为数字 说明:当c为数字0-9时,返回非零值,否则返回零*/    
						    	  	 if (isdigit(c) )
									    	  	  {
								    	  	    	  i = 0;
								    	  	    	  s[++i]=c;
									               while (isdigit(s[++i] = c = getch()))
									                    ;
									                  
									                  /*is a dot */
									              if (c == '.') /* collect fraction part */    
									                while (isdigit(s[++i] = c = getch()))
									                      ;
									               s[i] = '\0';
									               if (c != EOF)
									               ungetch( c);
									              return NUMBER;
									    	  	  }
								    	 	else
								    	 	{
								    	 	
								    	 		 printf("this is no a number\n");
								    	 	  	return '-'  ; /* not a number */
								    	 	}
						   }
						   
							 if (!isdigit(c) && c != '.')
							     return c; /* not a number */	     
							  i = 0;
							  if (isdigit(c)) /* collect integer part */
							/* isdigit()功能:判断字符c是否为数字 说明:当c为数字0-9时,返回非零值,否则返回零*/
							  while (isdigit(s[++i] = c = getch()))
							         ;
							  if (c == '.') /* collect fraction part */
							     while (isdigit(s[++i] = c = getch()))
							             ;
										 s[i] = '\0';
										 if (c != EOF)
										ungetch( c);
							
							   return NUMBER;
				}